This is a 6mm Heat shrinkable tube. Heat shrink tubing (or, commonly, heat shrink) is a tube which shrinks in diameter when heated. Shrinks to 50% of its diameter core after heat is applied to it with the help of heat gun.
Heat shrink sleeve containing a completely thermoplastic covered interior surface.
On heating, the diameter of the sleeve reduces by 2:1, and the interior resin melts. On cooling, the sleeve and the resin harden. This combination guarantees resistance to pullout and a perfect waterproofing for your connections, the whole preventing a sleeve from slipping. In the case of a connector mounted on a cable (coaxial for example), this sleeving ensures a reinforcement at the place where the cable is generally subject to stress when the radius of the curve is low.